The present invention relates to a method for laser beam welding components that are composed of a metal matrix composite material to form a full bond strength and a uniformly dispersoid structure in the weld bead. Metal matrix composites, despite their high strength and modulus to weight properties, are restricted in application because of the inability to maintain their structure during welding. A similar problem occurs with particulate containing materials, e.g., Oxide Dispersion Strengthened [ODS] alloys. In both cases the lighter particles tend to float out of the melt during the liquid phase of a conventional welding process. The weld zone is, therefore, depleted in the strengthening agent which consequently leads to inferior properties in the weld zone. Examination of metal matrix composite [MMC] joint properties often shows that these strengths are only compared to that of the matrix alloy since the joint properties are so poor in comparison to those of the composite.
Examination of a conventional arc welding process demonstrates why segregation of the species occurs. In gas metal arc welding (GMAW) the heat of the arc melts both the base plate material and the consumable wire electrode (filler wire). In a typical case the liquid phase weld pool, that is formed by the base plate and the material that has been transferred across the arc from the filler wire, will be approximately one half an inch in diameter and one eighth to one quarter inch deep. Within this superheated pool, flotation of the lighter, inert species will occur. Subsequent solidification of the molten bead can only entrap the particles or fibers at the position to which they have floated, i.e., on the surface. Furthermore, in GMAW the filler material is actually transferred to the pool in liquid (droplet) form. In this case it may be anticipated that the use of metal matrix composite materials as filler wire will not only result in phase separation but also probably in seriously unstable welding arc conditions, in addition to the loss of properties caused by the separation of the phases.
Filler metal additions may also be made to gastungsten arc welding (GTAW) processes by introducing a filler wire into the weld pool. When the wire is heated by the passage of an electric current, this is known as "hot wire gas-tungsten arc welding" (HWGTAW). Conventional hot wire GTAW suffers from the need to keep the filler wire in contact with the electrically grounded workpiece and is, therefore, somewhat difficult to control. Specifically, problems arise when the heat from the arc source is sufficiently intense to melt the wire before it reaches the weld pool: in this case the electrical continuity required to heat the wire is lost and the control circuit becomes unstable. Also, while such a process, involving hot or cold wire, appears to be a reasonable approach to introducing the composite material into the weld zone in a solid, and thus, with a relatively uniform distribution of particles or fibers, the weld pools that are developed in GTAW are still of a large size. The size of the pool, and the high temperatures that are developed across the pool, allow mechanical separation of the strengthening and matrix phases by flotation before solidification.
In arc welding processes the energy density that is provided by the heat source in order to melt the materials is sufficiently low to require an overall large heat input and, in turn, to produce a large pool size. Laser beams offer the possibility of much higher energy densities and consequently smaller weld pools. Moreover, in laser beam welding the travel speeds attainable are generally much higher than in arc welding. This results in a much more rapid solidification process of the weld zone and might permit the retention of a suitable phase distribution. Unfortunately the most often used mode of laser beam welding is the "Keyhole" mode in which the laser beam forms a cavity around the surface of which molten metal flows, to solidify after the passage of the beam and its cavity. In this mode the disadvantages relative to welding of metal matrix composite components are:
1. The temperatures within the cavity are extremely high so that any material introduced into it will be melted rapidly and flotation and separation could occur in metal matrix composite materials;
2. The cavity temperatures are so high that vaporization loss of volatile elements that are required for matrix strengthening can occur, e.g. Mg in Al-based metal matrix composites; and
3. The cavity is of such a small diameter that feeding of a wire into the cavity is very difficult to control.
An alternative mode of laser beam welding is available. In the "conduction" mode of laser melting the beam power is insufficient to form a cavity and only a small surface layer is heated. Rapid passage of the beam across an area of a metal surface results in a very thin layer of material melting rapidly under the beam action and then solidifying rapidly as the excess heat is conducted away into the body of the material. It has previously been demonstrated that the use of conduction mode welding allows the addition of filler metal additions to laser beam welds. In one method, a special in-line resistance heating conduit is used to direct the filler wire to the weld pool. In this method a line or rectangular shaped beam spot directed onto the weld seam ahead of the wire to provide a thin film of liquid substrate into which the already heated filler wire can be added.
A method of laser beam conduction welding of metallic components using a filler material is described in the aforementioned copending related U.S. Pat. No. 4,737,612, the contents of which are incorporated by reference herein. As described therein, specifically in relation to FIGS. 6 and 7 thereof, a laser beam With a larger waist, and thus lower power density, than that used for keyhole welding, is directed at adjacent portions of one or both of the metallic components to be joined. This produces a conduction weld of molten pool of metal. Heated filler wire is fed into the pool to add filler material to the weld as the components are moved relative to the laser beam. A single pass is made along each edge of the confronting metal components to be joined to produce a finished weld, with a full penetration weld resulting.
It is an object of the present invention to provide a method of laser beam welding of metal matrix composite components that provides a uniform particle or fiber strengthened metal matrix weld joint.
It is another object of the present invention to provide a method of welding metal matrix composite components with full strength joining of the metal matrix composite components.